This request for an ADAMHA Research Scientist Development Award focuses on hormonal (gonadal and pineal hormones), environmental (e.g., photoperiod and temperature), and dietary influences on energy balance and reproduction in rodents. There are two lines of research which focus on different aspects of these problems. The overall goal of the first line of research is to determine the mechanisms by which gonadal steroids (estrogens, progestins, and androgens) influence energy balance, body weight, and body composition. Gonadal hormones affect nearly every aspect of energy metabolism including energy intake (total caloric intake and dietary self-selection), partitioning of circulating metabolic fuels among various tissues and organs, and energy expenditure (e.g., voluntary exercise and thermogenesis). Gonadal effects on energy balance are widespread among mammalian species ranging from rodents to primates. One group of experiments will focus on sites and mechanisms of hormone action in rats. Some of this work will examine and characterize gonadal hormone actions in brown adipose tissue, an important site of thermogenesis and energy expenditure. Other experiments will investigate hormone effects on energy balance in Syrian and Siberian hamsters, two species which differ from rats in their responses to steroids. The objective of the second line of research is to examine the effects of diet and environmental factors such as photoperiod on energy balance and adiposity in Syrian hamsters. Hamsters fed a high-fat diet are unusual among animal models of obesity. They do not overeat, but they become obese because of decreases in energy expenditure. This decrease in actual energy expenditure is accompanied by an increase in the capacity for thermogenesis and brown adipose tissue. All of these effects of high-fat diets are exaggerated by exposure to short photoperiods, the principal environmental cue for seasonal change in hamsters. One group of experiments will further characterize the high-fat diet-induced obesity and will examine the effects of exercise and environmental temperature. A second group of experiments will focus on the effects of photoperiod and the pineal gland hormone, melatonin (which may mediate the reproductive effects of photoperiod), on energy balance and brown adipose tissue in hamsters. This interdisciplinary approach should yield new insights as to how factors such as hormonal condition, diet quality, and seasonal cues (including photoperiod and temperature) can interact to affect behavioral and metabolic regulation of body weight.